Double neck wiring question

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Double neck wiring question

Post by Rock64 »

Hello, First post

I have a double neck with a piezo in the top side, and magnetic pickups on the other. I'd experimented for a long time with two sets of wires hanging off the guitar into two separate destinations. Once I was happy with my arrangement I decided to wire it up to a stereo jack and use a TRS cable divided at the end.

Problem I'm having now is that despite absolutely zero contact with each other, (the exception being the ground), I'm getting a 90/10 bleed both ways from the guitars. This is happening in the guitar as I have tested the continuity of every piece of the chain many times over and the crossover bleeding is inside the guitar. I test the stereo jack, cable, and the splitter, and all of them combined before soldering and it's 100% perfect. As soon as I solder the leads I have bleeding. I'm getting bleed across the A and B. Just to be clear, I have been testing this arrangement for a year and the signal path works as intended so my gear settings and routing are not the issue.

I have two totally discreet signal paths (The exception being the ground). No matter how I wire it it bleeds in some way back and forth. I thought a piezo was to blame but I tried a single coil instead and still it bleeds. In addition, I have tried soldering both sides directly to the jack with no other connections and it still bleeds.

Is it possible that I'm mistaken in assuming you can run two separate signals through one cable (stereo) and with a common ground have no bleed? If that were the case then my headphones would forever be mono.

Imagine for a moment you had an SG with it's two volumes and two pickups. Imagine switching to the bridge. The neck pickup has no chance of coming through. Now imagine you took that guitar and instead of a 3 way switch, you wired both pickups directly to a stereo jack and all switching control was outside the guitar, but when you cut out the signal from the bridge pickup you still hear it coming through the neck side. This is what is happening to me. What on earth am I doing wrong?
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Re: Double neck wiring question

Post by Hugh Robjohns »

I suspect the problem you're struggling with is crosstalk within the cable due to the very high impedances of the pickup sources and of the destination amplifier inputs.

This crosstalk isn't such a problem with headphones because the impedances involved are hundreds of Ohms at most, and often only tens of Ohms. In the guitar you're looking at hundreds of kilo-Ohms or even Meg-Ohms. These very high impedances means that the inherent small capacitances between signal wires within the cabling act to couple the sound across quite efficiently.

Using a cable with two individually screened cores would undoubtedly help, but I expect you'll either need physically separate outputs and two separate cables (as per your trial rig), or active buffering circuitry inside the guitar to convert from the high-impedance internal wiring to provide low impedance outputs.
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Re: Double neck wiring question

Post by Wonks »

Hi and welcome.

That sounds very strange. Of course, there is a possibility of the signal transmission from one 'hot' cable to another 'hot' cable within the TRS cable itself. The high impedances of guitar pickups and piezo circuitry make this far more likely than with the much lower impedances involved in headphones, though there still is some measurable bleed in headphones between channels - which is why some headphones use a 4-wire system.

The longer your TRS to TRS lead, the bigger the cross-talk signal is going to be. Have you got a short TRS to TRS patch lead that you can use between the guitar and the splitter box to see if the cross-talk reduces significantly?
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Re: Double neck wiring question

Post by Rock64 »

Hugh Robjohns wrote: Fri Mar 31, 2023 3:18 pm I suspect the problem you're struggling with is crosstalk within the cable due to the very high impedances of the pickup sources and of the destination amplifier inputs.

This crosstalk isn't such a problem with headphones because the impedance's involved are hundreds of Ohms at most, and often only tens of Ohms. In the guitar you're looking at hundreds of kilo-Ohms or even Meg-Ohms. These very high impedance's means that the inherent small capacitance's between signal wires within the cabling act to couple the sound across quite efficiently.

Using a cable with two individually screened cores would undoubtedly help, but I expect you'll either need physically separate outputs and two separate cables (as per your trial rig), or active buffering circuitry inside the guitar to convert from the high-impedance internal wiring to provide low impedance outputs.

Thanks for the quick reply. I think this cable is about 10 ft. I had a 3 footer that did exactly the same thing. I did try placing a homemade 9v eq circuit I had here on the electric side and that seemed to stop the piezo side from making its way into the electric side. I think I tried it on the piezo side and it still bled. I might have to try that again to be sure.

Fishman powerchip claims to be able to bring a magnetic and piezo pickup together or separate them to a stereo trs cable but that lil thing is 130 bucks.

So I'm correct in assuming that this passive arrangement is not going to work as is? There need to be some type of buffering involved if I don't wan the two signals to push their way into each other? And this all happens through the common ground. Why then didn't they do this when the two sets of wires I used to have, hit the common ground of my gear? I place a meter across those contacts and they're a cleaner continuity than anything I've tested.
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Re: Double neck wiring question

Post by Hugh Robjohns »

Rock64 wrote: Fri Mar 31, 2023 4:24 pmI did try placing a homemade 9v eq circuit I had here on the electric side and that seemed to stop the piezo side from making its way into the electric side.


Yes, I'd expect that to work because it provides a low impedance output to drive the cable.

I think I tried it on the piezo side and it still bled. I might have to try that again to be sure.

Try it again, as it should work!

So I'm correct in assuming that this passive arrangement is not going to work as is?

Only if you use separate cables.

There need to be some type of buffering involved if I don't wan the two signals to push their way into each other?

That would certainly be the best way.

And this all happens through the common ground.

No, it happens because of capacitively-coupled crosstalk between the two output wires inside the cable due to the very high-impedance environment.

Why then didn't they do this when the two sets of wires I used to have, hit the common ground of my gear?

Because they were physically separate wires, so the capacitive coupling was negligible. The common ground is completely irrelevant.
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Re: Double neck wiring question

Post by Rock64 »

Thank you for the crazy fast replies, but most of all for the truly helpful details and concise pinpoint answers. I'm making people mad elsewhere as they are trying to help but aren't getting it.
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Re: Double neck wiring question

Post by Sam Spoons »

I'm guessing this is a problem because you have two amps with the mag pickups bleeding into the acoustic amp and the piezo bleeding into the electric amp? Can you not prevent the signal going to the amp you are not using with an A/B/Y box?
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Re: Double neck wiring question

Post by Rock64 »

Sam Spoons wrote: Fri Mar 31, 2023 5:19 pm I'm guessing this is a problem because you have two amps with the mag pickups bleeding into the acoustic amp and the piezo bleeding into the electric amp? Can you not prevent the signal going to the amp you are not using with an A/B/Y box?

Once the two signals leave the guitar thru the trs, they're split and sent into two separate inputs on a pedal board and the expression pedal is programmed to pan between two inputs. This worked beautifully and still does but the bleed ruins the 100% end product I used to get with two separate cables. If that makes any sense.
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Re: Double neck wiring question

Post by Sam Spoons »

Yes it does. Why do you need to pan between the two signals, do you ever blend them together (not sure how you could given that that would have to play both necks at once?). Can you gate the two inputs so that when you reach say 15% the gate activates to silence the bleed?
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Re: Double neck wiring question

Post by Wonks »

Rock64 wrote: Fri Mar 31, 2023 5:24 pm Once the two signals leave the guitar thru the trs, they're split and sent into two separate inputs on a pedal board and the expression pedal is programmed to pan between two inputs. This worked beautifully and still does but the bleed ruins the 100% end product I used to get with two separate cables. If that makes any sense.

If you aren't playing the piezo neck, how does it produce any sound if you are just playing the magnetic neck (apart from sympathetic vibrations being picked up)? Or do you alternate between the two necks quickly? Or are you doing something like tapping on both the necks together at times?

I could understand more easily it if it was a single neck with piezo and magnetic pickups.

And presumably within the pedalboard/multi FX you've got one set of FX treating the acoustic sound and another the magnetic sound, and the bleed is affecting the tone of the signals.
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Re: Double neck wiring question

Post by Rock64 »

Sam Spoons wrote: Fri Mar 31, 2023 5:30 pm Yes it does. Why do you need to pan between the two signals, do you ever blend them together (not sure how you could given that that would have to play both necks at once?). Can you gate the two inputs so that when you reach say 15% the gate activates to silence the bleed?

Last question first. The bleed is in the guitar signal so I cannot affect by that point. Its literally coming through the wrong side all the time.

I used to play just one guitar onstage and if ever needed an acoustic sound I simply used a sim. That was fine for what I did, but I always wanted to do more. I just didn't believe the sims. I once played two guitars strapped on my carcass, one electric and my acoustic. It was a simple tune and it worked. When the other guitarist quit I had to condense so the simulator became king.

I've been toying with this idea for a very long time and I convinced the wife to buy me this doubleneck for Christmas. I actually paid for it but she's run out of gift ideas for me so it was win win. I've become totally dependent on expression pedals for almost anything onstage. I make them do 10 things at once so I don't have to look down for buttons and I can't get forward/backwards wrong. I can feel on and off or anything in between.

I don't always want to pan but sometimes I find a little cross-fade is nice with this arrangement as I'm not two guys with 4 hands. I never play both at once and never foresee wanting that but you never know. I could do that too if need be with this arrangement.
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Re: Double neck wiring question

Post by Rock64 »

Wonks wrote: Fri Mar 31, 2023 5:42 pmIf you aren't playing the piezo neck, how does it produce any sound if you are just playing the magnetic neck (apart from sympathetic vibrations being picked up)?

You got it. The biggest problem is the electric contaminating the acoustic. I use a string damper to quell that. I had the acoustic vibration problem under control but now it's sneaking in through the electronics and that is why you find me here :D

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Re: Double neck wiring question

Post by Wonks »

A two- or three-position neck selector switch would stop the bleed issue. Has it got one and you don’t like using it or if not, is there somewhere you could fit one?
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Re: Double neck wiring question

Post by Sam Spoons »

I'm now very curious :) what exactly is the doubleneck (photo maybe)? Just to prove it I've tested a mic cable with 2 x TS to XLR adapters both ends and there is significant crosstalk when plugging unconnected jacks into a guitar and my Headrush fx unit. There must be a way to make it work, can you program the fx unit to roll off the volume on the inactive input as it pans between the two?

Wonks, a two way selector might work if it shorted the inactive output to earth but I don;t think it would solve this particular problem if it left it floating in the normal way as the crosstalk is being induced in the cable?
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Re: Double neck wiring question

Post by Wonks »

The signal has to reach the cable for crosstalk to occur. Floating should be good enough, though grounding would be better. But for your standard easy-to-flick guitar 3-position Switchcraft-style switch, grounding wouldn’t be possible.

That size of DPDT switch that could do the grounding tend to be rather clunky and stiff.
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Re: Double neck wiring question

Post by Sam Spoons »

But the signal causing the problem is from the live side of the guitar? But yes, I get your point about switches capable of grounding the pickups.
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Re: Double neck wiring question

Post by Wonks »

Switchcraft do a DPST+DPST pickup selector switch which could be used to ground the signal of the non-selected side. But in the mid-position you wouldn’t get any signal. They aren’t that easy to find either and are naturally bulkier than a standard switch. They used to do a lot more switch variations (including ones with changeover contacts) but have cut right back fairly recently.

Because the acoustic and electric halves were previously fully isolated, with separate jack outputs, there wasn’t any need for a neck selector switch. But now there’s a common TRS jack output and crosstalk from the TRS cable, a selector switch would be very useful.

With no magnetic signal flowing down the T or S cable connection, there won’t be any magnetic signal crosstalk to the piezo side. If left floating, the piezo signal might impinge on the magnetic side, but probably something a very low threshold noise gate could take care of. But when selecting the piezo neck, you wouldn’t get any magnetic pickup interference on the piezo signal.

And vice-versa.

But knowing what the guitar is would help in working out whether there’s anywhere a switch could go.

A 2-position mini-toggle switch could probably fit somewhere, but they are never quite as easy to use as a standard pickup selector.

I suppose that a push/pull or push/push volume or tone pot could be used for the neck selection, which would allow the grounding of the unused output.
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Re: Double neck wiring question

Post by Sam Spoons »

I think the OP prefers to do his neck selection from the pedal on his fx unit so any suggestions for on guitar switching are probably moot but I'm sure something could be done.
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Re: Double neck wiring question

Post by Wonks »

Well, I think it’s probably a choice between
a) adding some buffers and a battery into the guitar (you can get DIY buffers kits for under £20/$25 each which would be cheaper than a Fishman kit) but you need to find or make the space for them and a battery, or
b) adding a selector switch, or
c) going back to twin outputs and cables.

The last option is the easiest and cheapest solution.

But someone may think of something else.

Does anyone make a stereo guitar transmitter?
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